In recent years, as a major technology trend in transformers and rotating machines, energy saving and efficiency enhancement have been underway including, for example, establishment of standards prescribing efficiency improvements. In particular, to reduce no-load loss, which is power loss caused in the iron core, i.e., so-called “iron loss,” each maker is committed to improvements of iron core materials and improvements of iron core structures.
Under these circumstances, for example, so-called grain-oriented electrical steel, which is intended to reduce loss, is adopted for the iron cores of transformers. As measures to make such improvements, measures such as high orientation impartment, magnetic domain refinement control, and reduction of plate thickness have been taken.
Among others, the magnetic domain refinement control is the latest iron loss reduction technology developed in Japan. That is, so-called grain-oriented electrical steel has a crystal structure, for example, as shown in FIG. 16, and each crystal is structured to have fine magnetic domains as shown, for example, in FIG. 17. Here, under a condition in which no magnetic field is applied externally to the grain-oriented electrical steel, magnetic domains parallel to each other have magnetic fields oriented in directions different from each other. Consequently, the magnetic fields cancel each other in the grain-oriented electrical steel as a whole, curbing iron loss.
However, when a magnetic field is applied externally to the grain-oriented electrical steel, magnetic domain walls which are boundaries of magnetic domains move, and a region magnetized in the same direction as the external magnetic field spreads. This makes it impossible to cancel out the magnetic fields in the grain-oriented electrical steel as a whole and consequently, iron loss cannot be curbed. In particular, when grain-oriented electrical steel is magnetized by an alternating current, a moving direction of a magnetic domain wall alternates, and thus energy involved in the movement of the magnetic domain wall tends to result in iron loss.
Here, a definite correlation has been recognized between iron loss and moving velocity of the magnetic domain wall, and higher the moving velocity of the magnetic domain wall, the larger the iron loss. Thus, paying attention to this point, magnetic domain refinement control finely divides a magnetic domain in a width direction, which is a direction at right angles to a rolling direction to reduce a distance traveled by the magnetic domain wall in 1 cycle of alternating current. This makes it possible to reduce the moving velocity of the magnetic domain wall and curb iron loss. Note that regarding techniques for finely dividing a magnetic domain, methods put to practical use include a method which applies local thermal stress by irradiating a surface of grain-oriented electrical steel with laser or plasma and a method which applies mechanical stress to a surface of grain-oriented electrical steel using gears. Generally, such a technique is called a scratching process.
The conventional scratching process, which is intended to finely divide each magnetic domain in one direction along the width direction of the magnetic domain, involves forming linear laser marks in one direction as disclosed, for example, in Patent Literatures 1 to 6. Consequently, since the magnetic domains can be finely divided only in the width direction, the process is effective in reducing iron loss during magnetization of the grain-oriented electrical steel in the rolling direction, but is not effective in reducing iron loss during magnetization in a direction at angles to the rolling direction. Therefore, if an iron core which has a part also magnetized in a direction other than a direction parallel to the rolling direction RD, e.g., in a direction TD, such as the width direction, perpendicular to the rolling direction RD, is made of grain-oriented electrical steel, an iron loss reduction effect is limited, where such iron cores include an iron core of a three phase transformer, an iron core with gaps, and an iron core of a rotating machine.